1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a two-pump hydraulic excavator that has two hydraulic pumps as hydraulic power sources for hydraulic actuators.
2. Description of the Related Art
FIG. 6 shows a hydraulic circuit of a hydraulic excavator including a boom cylinder 4, an arm cylinder 5, and a bucket cylinder 6.
In this circuit, first and second hydraulic pumps 7 and 8 are driven by an engine (not shown). The first hydraulic pump 7 drives the boom cylinder 4, the arm cylinder 5, and the bucket cylinder 6. The second hydraulic pump 8 drives the boom cylinder 4, the arm cylinder 5, and a swiveling motor 9.
Control valves for controlling operations of the hydraulic actuators are separated into two groups. A first group G1 includes a first boom control valve 10, a bucket control valve 11, and a second arm control valve 12, which are supplied with hydraulic power from the first hydraulic pump 7. A second group G2 includes a second boom control valve 13, a swiveling control valve 14, and a first arm control valve 15, which are supplied with hydraulic power from the second hydraulic pump 8.
The center bypass passages of the control valves of the groups G1 and G2 are connected to the hydraulic pumps 7 and 8, respectively, in series by tandem circuits 16 and 17, respectively. The pump ports of the control valves of the groups G1 and G2 are connected to the hydraulic pumps 7 and 8, respectively, in parallel by parallel circuits 18 and 19, respectively.
In order to prevent the oil discharged from the pumps from being supplied only to the arm cylinder 5, which has a relatively light load, during a combined operation of arm pulling and boom raising or bucket excavation (hereinafter simply referred to as “combined operation of arm pulling and boom raising”),    (i) the arm control valves 12 and 15 are disposed at the most downstream positions with respect to the pumps 7 and 8 in the groups G1 and G2; and    (ii) a throttle 20 is provided at the entrance of the second arm control valve 12 in the parallel circuit 18 of the first group G1.
Thus, during the combined operation, the oil discharged from the first hydraulic pump 7 is preferentially supplied to the boom cylinder 4 or the bucket cylinder 6, and operation of the cylinders 4 and 6 is ensured.
At this time, the oil discharged from the second hydraulic pump 8 is sent to the arm cylinder 5 via the parallel circuit 19 and the first arm control valve 15 of the second group G2. Therefore, a flow necessary for the arm cylinder 5 is secured. The control valves 10, 13, 11, 12, and 15 are controlled by a boom remote control valve 21, a bucket remote control valve 22, and an arm remote control valve 23.
Here, attention is focused solely on the combined operation of arm pulling, whose operating pressure is relatively low, and operation of an actuator whose operating pressure is higher than this (boom raising or bucket excavation in the circuit of FIG. 6). Therefore, to simplify the figure, the remote control valve for swiveling, which is unrelated to the focus, is omitted. In addition, of pilot lines connecting the remote control valves 21 to 23 and the control valves 10, 13, 11, 12, and 15, only a boom raising pilot line 24, a bucket excavation pilot line 25, and an arm pulling pilot line 26 are shown.
Reference numerals 27 and 28 denote pump pressure sensors that detect the discharge pressures (pump pressures) of the pumps 7 and 8. The sensors 27 and 28 send pump pressure signals to a controller 29. The controller 29 sends control signals for controlling the pump discharge amounts to proportional valves 30 and 31 serving as pump regulators. That is to say, in order to prevent engine stall, the pump discharge amounts are controlled according to the pump pressures (horsepower control). In FIG. 6, reference character T denotes a tank.
In this configuration, during a combined operation including arm pulling (the case of arm pulling and boom raising will be described), the oil discharged from the hydraulic pumps 7 and 8 is supplied to the expansion side of the boom cylinder 4 via the boom control valves 10 and 13, and to the expansion side of the arm cylinder 5 via the arm control valves 12 and 15.
At this time, the oil discharged from the first hydraulic pump 7 is throttled on the upstream side of the tandem circuit 16 of the first group G1, by the center bypass passage of the boom control valve 10. Therefore, extra heat is generated in this part.
In addition, due to the throttling in this bypass passage, the pump pressure increases. Therefore, the horsepower control is performed not only on the first hydraulic pump 7 but also on the second hydraulic pump 8, and the flow of the entire circuit decreases.
As a result, the amount of the oil supplied to the arm cylinder 5 decreases. Therefore, the operating speed of the arm 2 decreases.
As a remedy for this, in order to ease the throttling in the center bypass passage of the boom control valve 10, the opening of this bypass passage can be enlarged.
However, in this case, during the combined operation, oil is mostly supplied to the arm cylinder 5, which has a lighter load, and the boom cylinder 4 does not operate. That is to say, it is meaningless to dispose the boom control valve 10 on the upstream side of the tandem circuit 16 and to dispose the arm control valve 12 at the most downstream position in the tandem circuit 16.
In a known technique, instead of the throttle 20 in the parallel circuit 18, a flow control valve is provided so as to increase the amount of the oil sent to the arm cylinder 5 via the parallel circuit 18 during the combined operation (see Japanese Unexamined Patent Application Publication No. 9-177139).
If this technique is adopted, since the amount of the oil sent to the arm cylinder 5 via the parallel circuit 18 during the combined operation increases, the operating speed of the arm cylinder 5 can be increased. In addition, since the amount of the oil passing through the tandem circuit 16 decreases, extra heat generation can be controlled.
However, the flow control valve is much more expensive than the throttle 20. In addition, a control system therefor is necessary. Therefore, the cost is considerably increased.
Moreover, since the flow control valve needs to be newly incorporated into the circuit, it is difficult to apply this technique to an existing machine.